This invention relates to a filter for removal of particulate matter in an exhaust gas stream, and more particularly, to a diesel particulate filter (“DPF”) and a coating process for a substrate of a DPF.
In order to meet exhaust emission standards, which are becoming more and more stringent, sophisticated catalyst systems are in development. Especially for diesel engine exhaust emission control, much more complicated catalyst systems are increasingly required because of the lean condition and presence of particulate matter in exhaust gas stream. As these standard become increasingly tightened, exhaust treatment systems may include an oxidation catalyst for the conversion of CO and hydrocarbon to CO2, a diesel particulate filter (“DPF”) for the removal of particulate matter, and a catalyst such as a NOx adsorber to remove NOx from the exhaust stream. These specific functions are essential components in the treatment of diesel exhaust. In addition, the regeneration of DPF and/or NOx adsorber catalysts may require additional catalysts downstream for an efficient removal of pollutants from the gas stream. Thus a complete catalyst system for diesel engine exhaust gas might comprise of three to four bricks of catalysts to meet the emission standard. Such a complex system would be impractical due to high cost associated with the catalyst, the canning, the system integration, etc.
The flow restriction (backpressure) of emission treatment systems has significant impact on engine performance and fuel economy. In general, the lower the backpressure, the better the engine performance and fuel economy will be. In catalytic emission after-treatment systems with honeycomb catalysts, most of the flow restriction is created by the honeycomb catalyst. The flow restriction is even more prominent when the honeycomb substrate is changed from a channel flow device to a wall-flow device. Multiple catalyst systems, therefore, that include DPF type wall flow devices again become impractical due to the severe flow restriction across the system. That is, high washcoat loading, such as for increased NOx adsorber function, leads to high back-pressure increase on DPF type of substrate, which may deleteriously affect engine performance and fuel economy.